Feb 18, 2020

How to Avoid the Fiber Weave Effect in PCBs with 3D Printing

fiber weave effect in pcbWhen only a small number of designs ran at extremely high speeds (<1 ns rise times), most designers did not worry so much about the fiber weave effect in PCB design and manufacturing. With low bandwidth digital signals and at low GHz frequencies, the effects of a loose fiber weave were generally too small to notice unless interconnects were very long. With longer rise times of ~5 ns, it would take anywhere from 5 to 50 ft. of interconnect length for sufficient timing skew/jitter to accumulate on an interconnect and lead to failure—something that would never be observed in practice.

With more devices running at higher speeds and higher frequencies, however, the fiber weave effect now becomes a common problem designers must confront. More advanced materials allow designers to completely avoid the fiber weave effect, which could benefit advanced devices in a number of areas. In particular, certain 3D-printed materials do not suffer from fiber weave effects, making them ideal for advanced electronics.

Oscilloscope for measuring the fiber weave effect in PCB laminates.

The fiber weave effect in PCB laminates creates interesting signal integrity problems in the time domain and frequency domain.

What Is the Fiber Weave Effect in PCB Laminates?

The fiber weave effect in PCBs refers to several well-known signal integrity problems that occur at high frequencies. These problems occur due to the intrinsically inhomogeneous, anisotropic nature of PCB laminates and cores, which are made from woven glass impregnated with resin. The extent of these effects depends heavily on the glass weave style, size of the pores left in the weave, and the type of resin used for impregnation. These problems become noticeable at mmWave frequencies and can have a major effect in applications requiring extremely precise time, low EMI, and low insertion loss.

The fiber weave pattern in a PCB substrate causes the dielectric constant in the substrate to vary along the length of an interconnect as the interconnect spans across glass-dominated and resin-dominated regions. Certain glass weave styles (such as 1067 and 1086 glass cloth) produce less-noticeable effects as the glass weave is than other weave styles (such as 106 and 1080 glass cloth). However, even these tighter weave styles will produce the same signal effects as loose glass weaves, albeit at higher signal frequencies.

Skew and Jitter

Because of the variation in dielectric constant throughout standard PCB laminates, there are slight variations in the impedance and signal propagation velocity along the length of an interconnect. This causes dispersion and skew to accumulate along the length of an interconnect, and the magnitude of both effects depends on the interconnect length and routing angle across the substrate. 

At high frequencies and on long interconnects, the skew and phase noise/jitter can accumulate to the point where it is comparable to the rise time of a digital signal. This problem with skew and jitter is normally addressed by routing traces at a slight angle (2° to 5°) compared to the substrate fiber weave direction.

Cavity Resonances

There is another problem that tends to occur above ~40 GHz, where resonances are excited in cavities in the glass weave. Cavities in a standard glass-weave laminate are semi-closed structures, and resonances can be excited as signals travel along an interconnect. These resonances can radiate strongly, acting as a source of radiated EMI from the cavity edge. Due to coupling with other circuits and structures in a standard PCB, resonance in a fiber cavity is known to excite subharmonic resonances, effectively down-converting high-frequency EMI to lower-frequency EMI.

This problem may not cause an EMC test failure when signal levels on the relevant RF interconnect are low. Any coupling to a nearby digital interconnect may also not cause the signal to break through a receiver’s noise margin when working with 3.3 V or 5 V ICs. However, RF power amplifier circuits in radar systems, 5G modems, and related applications can see strong subharmonic coupling between circuits as these devices run at high power.

Analog signals on an oscilloscope.

This type of signal degradation can become prominent in high power RF devices due to the fiber weave effect in PCB substrates.

How 3D-Printed Polymer Substrates Eliminate Fiber Weave Effects

Any substrate material that has some inhomogeneity in its structure will create a similar effect as the fiber weave effect in standard PCB laminates. Polymer substrate materials do not experience these effects until well into the THz range, making them an ideal choice for use in high-frequency and high-speed devices. This eliminates issues with skew and cavity resonances in these devices. This is because these materials are flat and optically homogeneous once cured.

Using a 3D printer to fabricate PCBs with insulating polymer substrates provides multiple benefits beyond eliminating the fiber weave effect in PCBs. A variety of polymer materials can be 3D printed from nanoparticle suspensions in an aerosol jetting, laser sintering, or inkjet deposition process. 

The advantage of inkjet printing is that conductors and a polymer substrate can be deposited simultaneously and cured as a single unit. This allows a complex multilayer structure to be built up in a layer-by-layer deposition process. 3D-printed PCBs that are fabricated in this way can have any interconnect architecture, planar or nonplanar board geometry, and complex shapes that are difficult to fabricate with standard processes.

3D-printed interconnect structures for high-frequency AMEs.

Example interconnect architectures that can be fabricated with an inkjet 3D printer.

The example interconnects shown above can be easily 3D printed with an inkjet system. These interconnects will have highly consistent impedance throughout their length thanks to the uniformity of the surrounding substrate. These types of interconnects also have lower roughness losses as these structures do not need to be fabricated with a copper etch process, in contrast to standard multilayer PCB fabrication processes.

The other benefits provided by polymer substrates for PCBs come through functionalization. The electrical and optical polymers can be easily tuned by adding functional groups on polymer chains and doping with impurities. These modifications can be performed in solution for several polymers at low temperature and standard pressure, as well as with readily available materials. This provides a simple way to optimize losses and parasitics in high-frequency interconnects for many unique applications and quickly fabricate these advanced devices.

If the fiber weave effect in PCB substrates is wreaking havoc on high-speed and high-frequency signal integrity in your devices, you can get much more accurate results when you use a 3D printer for your high-performance additively manufactured electronics (AME) device. The DragonFly LDM system from Nano Dimension is ideal for high-mix, low-volume in-house production of planar or nonplanar additively manufactured electronics (AMEs) in hours. This advanced system can be brought into a production line alongside standard PCB assembly equipment and other additive systems for enclosure fabrication. Read a case study or contact us today to learn more about the DragonFly LDM system.

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